1. Field of the Invention
The present invention relates generally to improved keys for cylinder locks and, more particularly, to bitted keys having increased strength to prevent their breakage.
2. Description of Relevant Art
Yale type cylinder locks are well known and have been in continuous use since 1865. Typically, in such locks key differs (i.e., bitting combinations) are developed by adjusting incrementally the lengths of the lower pin tumblers and the corresponding depths of the bitting surfaces on the key blade. U.S. Pat. Nos. 3,499,302 and 3,722,240 to Spain et al. (1970) disclosed tumbler pins with chisel-shaped tips which were rotatable to predetermined positions by skew cut bittings on a key, the tumbler pins operating in conjunction with a side bar mechanism to provide the lock with a dual locking feature. For example, see FIG. 1 of the present application which illustrates such a high security cylinder lock manufactured by Medeco Security Locks, Inc. The aforesaid patents greatly improved the state of the cylinder lock art by teaching tumbler pins which, in addition to the traditional elevational positioning, were rotationally positioned. This significantly increased the number of unique key differs, for example, from 1,000,000 to 729,000,000 in a lock having six tumbler pins.
Increasing the number of key bitting increments results in a higher number of unique key differs. This greatly reduces the potential for any key operating a cylinder other than its own. Patents issued for the great majority of lock cylinders on the market have expired. Their keys can be copied on conventional machines of the type described in U.S. Pat. No. 1,439,382. The keyblanks required are widely distributed beyond the control of the lock manufacturer. The development of skew cut bittings provided additional security to the key owner because conventional rotary machines would not duplicate these angled cuts. As there have been machines developed to duplicate skew cut bittings, their security has been reduced. Uniquely shaped bittings and controlled distribution of proprietary keyblanks reduces the odds that keys in the possession of dishonest employees can be copied at hardware stores and the like. Notwithstanding improvements in the well worked locking arts, there remains a continuous need for mechanisms which can provide an extraordinary number of unique keys which are not subject to unauthorized duplication.
With the advent of elevational and rotational positioning of tumbler pins within the lock cylinder, the shapes of the pin tips and corresponding key bitting surfaces became a significant factor in the operation of the lock. For example, skew cut key bittings forced the tumbler pins to rotate to a precise position to permit operation of the lock. Irregularly or imprecisely configured surfaces on either the pin tips or the key bittings prevented proper positioning of the pins and operation of the lock. The desire in the art to produce numerous key differs has resulted in other problems as well.
One problem which has arisen is that the strength of the key blade is sometimes compromised as a result of the bittings cut therein. Specifically, the bittings are cut in the key blade at various depths and, as such, some areas of the key blade have more material removed (and thus are weaker) than other areas of the blade. The deep cut bittings which extend a considerable distance into the thickness (cross-section) of the key blade require removal of a substantial volume of blank material and produce significantly weakened areas of the blade. This has resulted in key blades breaking at the weakened areas, for example, due to the forces the blade is subjected to when inserted into the keyway of a lock and torqued by a user grasping the key bow.
FIGS. 2A-2B of the present application illustrate a conventional key with skew cut bittings 10 extending from one side surface 11 of the blade to the other side surface 12 through the thickness of the key blade, i.e., from the upper surface toward the bottom surface in FIG. 3. The bitting nearest the key bow (FIG. 2B) is cut at a deep level, i.e., it extends well toward the bottom edge of the key blade. As seen in the sectional view of FIG. 3, such a deep bitting leaves only the cross-sectional area 13 to provide structural integrity to this area of the key blade. Additionally, major warding groove 15 located below the bitting area significantly reduces the cross-sectional area of the key. The strength of the key blade, however, is not compromised in the areas of the blade which have shallow bittings, such as bitting 10 located near the end of the blade opposite the bow. As seen in FIG. 4, a considerable amount of blank material 14 remains after cutting this bitting into the key blade.
Cylinder locks having horizontal keyways which receive a horizontal flat key are known in the art and the problems discussed above pertain to such flat keys as well. As seen in FIGS. 6-8, a flat key has a rectangular shape with two short sides 31, 32 connecting larger upper and lower flat surfaces, the width W of the key being greater than the thickness T of the key. Flat keys for horizontal keyways are bitted on the large flat upper and/or lower surfaces. The bittings 41, 42 typically comprise recesses which are drilled into the flat surface(s) of the key blade. See, for example, U.S. Pat. Nos. 4,289,002, 4,553,452 and 4,964,288.
FIGS. 7 and 8 illustrate, respectively, deep cut and shallow cut bittings 41 and 42 in a flat key. The deeper bittings (FIG. 7) necessitate removal of a substantial amount of the thickness T of the blade. Also, the bittings become wider as they are cut deeper so as to extend further into the thickness of the blade, which removes even more material and further weakens the key. In the case of a reversible flat key bitted on opposite surfaces (FIG. 7), the blank material remaining to provide strength and prevent breakage of the key is further reduced. Additionally, major grooves 35 significantly reduce the cross-sectional area of the key. Accordingly, the bittings depicted in FIG. 7 significantly reduce the cross-sectional strength of the flat key blade which leads to breakage of the key blades in much the same manner as discussed above regarding standard keys bitted on the narrow upper edge of the blade.
It is evident that the placement of deep bittings, for example such as those shown in FIGS. 3 and 7, results in a weakened area of the key blade which is susceptible to breakage or other failure; however, the placement of deep bittings in key blades is necessary in order to obtain the maximum number of key differs. Accordingly, there is a need in the art for improved keys which possess sufficient strength to prevent their breakage yet permit the placement of both shallow and deep cut bittings in the key blade.